1. Field of the Invention
The invention relates to an intra-aortic balloon pump condensation prevention system. More particularly, the invention relates to an improved system for preventing water vapor from condensing in the shuttle gas of an intra-aortic balloon pump system.
2. Description of the Prior Art
Intra-aortic balloon (IAB) catheters are used in patients with left heart failure to augment the pumping action of the heart. The catheters, approximately 1 meter long, have an inflatable and deflatable balloon at the distal end. The catheter is typically inserted into the femoral artery and moved up the descending thoracic aorta until the distal tip of the balloon is positioned just below or distal to the left subclavian artery. The proximal end of the catheter remains outside of the patient's body. A passageway for inflating and deflating the balloon extends through the catheter and is connected at its proximal end to an external pump. The patient's central aortic pressure is used to time the balloon and the patient's ECG may be used to trigger balloon inflation in synchronous counterpulsation to the patient's heart beat.
Typical dual lumen intra-aortic balloon catheters have an outer, flexible, plastic tube, which serves as the inflating and deflating gas passageway, and a central tube therethrough formed of plastic tubing, stainless steel tubing, or wire coil embedded in plastic tubing. A polyurethane compound is used to form the balloon. Helium gas is typically used as the "shuttle gas", the gas used to inflate and deflate the IAB. To be effective, the IAB inflation and deflation must occur rapidly, e.g. in less than one eighth of a second.
During operation of the IAB helium gas diffuses from the IAB to the patient and the atmosphere and water vapor diffuses from the patient into the IAB. The outer tube of the IAB is thin (approximately 0.004 inches thick) and is generally made from a mixture of polyurethane and silicone, materials permeable to helium and water vapor, and thus allows for the above mentioned diffusion. To compensate for the foregoing loss of helium, intra-aortic balloon pumping (IABP) systems replace or add helium gas on a periodic basis. If the helium is not replaced on a periodic basis then the balloon will not completely inflate and therapy can be diminished. If the helium is not replaced at all the balloon will simply not inflate.
To compensate for the introduction of water vapor into IABP system some IABP systems incorporate a water vapor removal device which removes or lowers the concentration of water vapor in the IAB's shuttle gas. If the concentration of the water vapor is not lowered, the water vapor will condense and appear as liquid water within the IABP's shuttle gas system. Over a sufficient period of time the water can accumulate and impede the flow of shuttle gas within the IABP system.
Condensation can be prevented if the dew point temperature of the shuttle gas is kept lower than the IABP's ambient temperature. Prior art IABP systems prevent condensate accumulation by using a thermo-electric cooler, such as a Peltier device. The cooler is used to keep a metallic block (a condensate trap) colder than IABP ambient temperatures. During IABP operation, the shuttle gas (helium) flows thru a drilling in the block. Due to the block's lower temperature condensate forms within the block and flows (due to force of gravity) into a sump. Periodically, the sump is automatically emptied via a valve. Shuttle gas is lost during the emptying process. Consequently, the sump is emptied concurrently with the shuttle gas removal or replacement.
The prior art condensate prevention systems have a number of drawbacks. First, emptying the sump causes a loss of shuttle gas, and therefore, the helium consumption of the prior art condensation prevention systems is higher. Furthermore, because the sump cannot be emptied without a loss of shuttle gas, emptying the sump requires a shutdown of the IABP system, and therefore, causes an interruption in therapy. Second, the thermoelectric cooler consumes a great deal of power cooling the condensate trap (the block), which is continuously warmed by the flow of shuttle gas. High power consumption is undesirable because a larger IABP system power supply, battery charger, and battery is required to accommodate such consumption. Third, the drillings within the condensate trap increase the dead volume within the shuttle gas system. During balloon inflation and deflation the velocity of the shuttle gas is very high. To maintain high IAB inflate/deflate speeds it is important to prevent unnecessary pressure drops in the shuttle gas circuit. To accommodate this design requirement the drillings within the condensate trap are made quite large. Unfortunately, the large drillings increase the dead volume in the shuttle gas system. Parasitic dead volume wastes IABP power and can reduce efficacy by-increasing the pneumatic compliance of the shuttle gas system. Fourth, the prior art systems require disassembly and replacement or sterilization of the condensate trap if blood enters the IAB. Occasionally, arterial plaque deposits abrade the IAB's membrane and thereby cause perforations in the membrane. When this occurs, blood can enter the IAB and may contaminate components within the shuttle gas system. In prior art systems, IAB blood penetration requires disassembly and replacement of the condensate trap. A final drawback of the prior art condensate prevention system is that it is orientation sensitive, i.e. the condensate trap relies on gravity. Orientation sensitivity of a system component limits the design flexibility of the entire IABP system.
The IABP condensate prevention systems discussed above are essentially devices which dry "wet" process gases. Perma-Pure Inc. manufactures a commercially available device for drying a "wet" process gas. Said device is generic, i.e. it is not made specifically for use with a specific machine or system, such as a an IABP system. Wet gas flows through a specially formulated plastic tube, made of Nafion, which is surrounded by a co-axial housing. The housing's interior, the area between the inner surface of the co-axial housing and the outer surface of the Nafion tube, is continuously swept with a dry "purge" gas. The purge gas and the process gases can differ in composition. As the wet process gas flows though the central tubing its water vapor is absorbed by the tubing wall and is transported to the purge gas. If the flow rate of the process gas is sufficiently low it is dried as it exits the assembly's Nafion tubing. A major disadvantage with the Perma-Pure drying system is that it requires a continuous source of dry process gas. A similar dehumidifier system using a membrane cartridge is disclosed in U.S. Pat. No. 5,681,368.
While the prior art designs may be suitable for the particular purpose employed, or for general use, they would not be as suitable for the purposes of the present invention as disclosed hereafter.